参考になったサイト

• http://www40.atwiki.jp/spellbound/pages/1620.html
• http://docs-jp.unity3d.com/Documentation/Manual/UnderstandingVectorArithmetic.html

検証用コード

• unityのHierachyViewに３つのオブジェクトを作成して確かめた

using UnityEngine;
using System.Collections;

public class TestDotProduct : MonoBehaviour {

	public Transform vec1;
	public Transform vec2;

	// Use this for initialization
	void Start () {
	}

	// Update is called once per frame
	void Update () {
		Debug.Log("vec1.position "+vec1.position);
		Debug.Log("vec2.position "+vec2.position);
		Debug.Log("transform.position "+transform.position);
		Vector3 OVec1 = vec1.position - transform.position;
		Vector3 OVec2 = vec2.position - transform.position;
		Debug.Log("OVec1 "+OVec1);
		Debug.Log("OVec2 "+OVec2);
		Debug.Log("OVec1.magnitude "+OVec1.magnitude);
		Debug.Log("OVec2.magnitude "+OVec2.magnitude);
		float dotFunction = Vector3.Dot(OVec1,OVec2);
		float dot = OVec1.x * OVec2.x + OVec1.y * OVec2.y + OVec1.z * OVec2.z;
		Debug.Log("dotFunction "+dotFunction);
		Debug.Log("dot "+dot);
		float cosTheta = dotFunction / (OVec1.magnitude * OVec2.magnitude);
		Debug.Log("cosTheta "+cosTheta);
		float radian = Mathf.Acos(cosTheta);
		Debug.Log("radian "+radian);
		float degree = radian * 180 / Mathf.PI;
		Debug.Log("degree "+degree);
		Debug.Log("");
		Debug.Log("");
	}
}

車の前方方向の速度を正確に取得するには以下の2つのURLを読む

• http://naop.jp/topics/topics14.html
• http://docs-jp.unity3d.com/Documentation/Manual/AmountVectorMagnitudeInAnotherDirection.html

法線／垂直ベクトルの計算

• http://docs-jp.unity3d.com/Documentation/Manual/ComputingNormalPerpendicularVector.html